The present invention relates to an optical device in which a surface of an element main body, such as an optical crystal or a lens, is given an anti-reflection processing.
In general, in an optical crystal or a lens the light intensity is diminished in consequence of the light reflection at the boundary surface. For instance, when the element body is formed of glass, the reflection loss at incident and emergent surfaces is, in total, about 8% for visible light. On the other hand, when it is formed of an optical crystal such as LiNbO.sub.3 which has a refractive index of 2.2 to 2.3, the reflection loss amounts to about 30%. In order to decrease the power loss due to the surface reflection, it is necessary to apply an anti-reflection treatment to the incident and emergent surfaces of the optical crystal or the lens. As such a treatment, it has been conventional to form a transparent thin film of oxide on the light incident and emergent surfaces by means of sputtering or electron beam deposition. For example, 1 to 3 layers of transparent thin film of oxide with thickness of about 100 nm are formed on the glass or the optical crystal such as LiNbO.sub.3. With the formation of such an oxide thin film, the reflection loss can be reduced to within 1%.
Now, when the oxide thin film is formed by the sputtering or the electron beam deposition, the surface of the optical crystal or the lens on which is formed the thin film is subjected to high temperature. For example, after the sputtering of 10 odd minutes, the temperature of the thin-film forming surface reaches higher than 200.degree. C. As a result, if the optical crystal or the lens is formed of a thermally vulnerable material, such as one having a phase transition point in the low temperature range, it is impossible to directly form the thin film on the surface of element body, so that the anti-reflection treatment can not be performed. Moreover, when the thin film has not been formed in the desired manner, it becomes necessary to remove the film by some suitable method to form a new film again. This results in a problem that more labor is required to provide the anti-reflection treatment with desired performance.